In recent years, aromatic polycarbonates have been widely used in various fields as engineering plastics having excellent properties with respect to heat resistance, impact resistance, transparency and the like. With respect to methods for producing aromatic polycarbonates, various studies have heretofore been made. Of the methods studied, a process utilizing an interfacial polycondensation between an aromatic dihydroxy compound and phosgene (the so-called “phosgene process”) has been commercialized, wherein 2,2-bis(4-hydroxyphenyl)propane (hereinafter, frequently referred to as “bisphenol A”) can be mentioned as a representative example of the aromatic dihydroxy compound.
However, the interfacial polycondensation process has problems in that it is necessary to use phosgene, which is poisonous; that it is necessary to use a very large amount of methylene chloride as a polymerization solvent, which is considered to be harmful to human health and the environment, wherein methylene chloride is used in an amount which is ten times as large as the amount of the aromatic polycarbonate produced; that a reaction apparatus is likely to be corroded with chlorine-containing compounds, such as by-produced hydrogen chloride, by-produced sodium chloride, and methylene chloride used as a solvent; that difficulties are encountered in separating and removing chlorine-containing impurities (such as sodium chloride and residual methylene chloride), which adversely affect properties of the aromatic polycarbonate produced; and that it is necessary to handle a large amount of waste water containing methylene chloride and an unreacted aromatic dihydroxy compound (such as bisphenol A).
As a method for producing an aromatic polycarbonate from an aromatic dihydroxy compound and a diaryl carbonate, a melt transesterification process has conventionally been known, in which an aromatic polycarbonate is produced by performing an ester exchange reaction between an aromatic dihydroxy compound (such as bisphenol A) and a diaryl carbonate (such as diphenyl carbonate) in the molten state, while removing an aromatic monohydroxy compound produced (such as phenol) from the equilibrium polycondensation reaction system. Contrary to the interfacial polycondensation process, the melt transesterification process has an advantage in that a solvent need not be used. However, the melt transesterification process has the following serious problem. The transesterification is an equilibrium reaction, and the equilibrium constant thereof is small. Therefore, the equilibrium reaction does not proceed unless the produced aromatic monohydroxy compound (such as phenol) is efficiently removed from the surface of a molten reaction mixture obtained by the transesterification. As the polymerization proceeds, the viscosity of a polymer being formed increases during the progress of the polymerization reaction and, hence, it becomes difficult to remove efficiently an aromatic monohydroxy compound (such as phenol) from the polymerization reaction system, thus making it difficult to achieve a high degree of polymerization with respect to a polycarbonate produced. That is, differing from the case of a melt polycondensation process for producing a polycondensation polymer (such as a polyamide or a polyester) other than an aromatic polycarbonate, the melt polycondensation process for producing an aromatic polycarbonate has the following problem. Even a prepolymer having a low degree (n) of polymerization (e.g., a prepolymer having an n value of from about 15 to about 20) has an extremely high melt viscosity and, hence, it is difficult to effectively facilitate the surface renewal of the prepolymer by usual agitation. Therefore, separation of an aromatic monohydroxy compound (such as phenol) from the surface of the polymerization reaction mixture does not occur, so that it becomes impossible to produce an aromatic polycarbonate having a high degree of polymerization (e.g., an n value of about 30 to about 65) which is required of an aromatic polycarbonate product for practical use. This problem is well known in the art.
Various polymerizers have been known for use in producing aromatic polycarbonates by the melt transesterification process. A vertical agitation type polymerizer equipped with an agitator is widely used for a small scale production in a laboratory. The vertical agitation type polymerizer equipped with an agitator is advantageous in that it exhibits high volumetric efficiency and has a simple construction, so that polymerization on a small scale can be efficiently carried out. However, when it is intended to use the vertical agitation type polymerizer for the production of an aromatic polycarbonate on a commercial scale, the following serious problem arises. When it is intended to use the vertical agitation type polymerizer for the commercial scale production of an aromatic polycarbonate, it is virtually impossible to effectively agitate the polymerization reaction mixture. Therefore, as mentioned above, it becomes difficult to efficiently remove an aromatic monohydroxy compound produced (such as phenol) from the polymerization reaction system, so that an aromatic polycarbonate having a desired degree of polymerization cannot be produced.
Specifically, a large-scale vertical agitation type polymerizer generally has a greater ratio of the liquid volume to the vaporization area than a small-scale one. In other words, the depth of a reaction mixture in the agitation type polymerizer is large and, hence, the pressure in the lower part of the agitation type polymerizer is high. In such a case, even if the degree of vacuum of the polymerization reaction zone is increased in order to achieve a high degree of polymerization, the polymerization proceeds under high pressure due to the weight of the reaction mixture in the lower part of the agitation type polymerizer, so that an aromatic monohydroxy compound (such as phenol) cannot be efficiently removed. Therefore, a large-scale vertical agitation type polymerizer is usable only in the production of a prepolymer having a low degree of polymerization. For obtaining a polymer having a desired degree of polymerization, it is necessary to subject the prepolymer having a low degree of polymerization obtained by using the agitation type polymerizer to a further polycondensation by using another polymerizer.
For solving the above-mentioned problem, various attempts have been made to remove an aromatic monohydroxy compound (such as phenol) from a high viscosity polymer being formed. Most of these attempts are concerned with improvement in mechanical agitation. For example, there are known a method using a screw type polymerizer device having a vent (see Examined Japanese Patent Application Publication No. Sho 50-19600 (corresponding to GB-1007302)); a method using an intermeshing twin-screw type extruder (see Examined Japanese Patent Application Publication No. Sho 52-36159); a method using a wiped film evaporation type reactor, such as a screw evaporator or a centrifugal film evaporator (see Examined Japanese Patent Application Publication No. Sho 53-5718 (corresponding to U.S. Pat. No. 3,888,826)); and a method using a combination of a wiped film evaporation type apparatus and a horizontal agitation type polymerizer (see Unexamined Japanese Patent Application Laid-Open Specification No. Hei 2-153923).
However, each of the above-mentioned methods mainly uses the technique of mechanical agitation and, hence, inevitably has a limitation accompanying the technique, so that it is impossible to completely solve the above-mentioned problem. Specifically, it is difficult to satisfactorily agitate a polymer having an extremely high viscosity by mechanical agitation and, hence, various problems encountered in the production of an aromatic polycarbonate from a prepolymer having an extremely high viscosity cannot be solved. On this point, a detailed explanation is given below.
In the above-mentioned method, it is attempted to lower the melt viscosity of the prepolymer as much as possible by elevating the temperature of the prepolymer. Specifically, the polymerization of the prepolymer is performed at a high temperature which is close to 300° C. under high vacuum while mechanically agitating the prepolymer so as to effect the surface renewal of the prepolymer. However, even at such a high temperature, the melt viscosity of the prepolymer is still very high, so that it is impossible to satisfactorily effect the surface renewal of the prepolymer. Therefore, by this method, the increase in the polymerization degree of the aromatic polycarbonate is inevitably limited and, hence, it is impossible to obtain a high molecular weight aromatic polycarbonate. Further, the method has the following disadvantage. Since the method is practiced at a high temperature which is close to 300° C., it is likely that the polymer produced suffers discoloration and lowering of properties. Further, it is likely that discoloration and lowering of properties of the polymer are also caused due to entrance of air and foreign matter into the polymerizer device through the vacuum-sealed gap between the casing of the polymerizer device and the rotary axis. Therefore, when it is intended to stably produce, by the method, a high quality aromatic polycarbonate for a long period of time, it is still necessary to solve various problems.
The present inventors completely solved the above-mentioned problems by developing methods which do not involve mechanical agitation. Specifically, the present inventors developed methods using a guide-wetting fall polymerizer device in which a molten prepolymer is allowed to fall along and in contact with the surface of a guide, such as a wire, thereby effecting polymerization of the molten prepolymer to produce a desired polymer (see Unexamined Japanese Patent Application Laid-Open Specification No. Hei 8-225641, Unexamined Japanese Patent Application Laid-Open Specification No. Hei 8-225643, Unexamined Japanese Patent Application Laid-Open Specification No. Hei 8-325373, W097/22650, Unexamined Japanese Patent Application Laid-Open Specification No. Hei 10-81741, Unexamined Japanese Patent Application Laid-Open Specification No. Hei 10-298279, WO99/36457, and W099/64492).
However, none of the above-mentioned patent documents has any teaching or suggestion about a method for producing an aromatic polycarbonate on a commercial scale at a rate of 1 t/hr or more. Further, it has been found that, even when the relatively small scale polymerizer devices disclosed in the above-mentioned patent documents are used for producing an aromatic polycarbonate for a long period of time, it is possible that the resultant aromatic polycarbonate product sometimes contain a very small amount of a polymer mass having too high a molecular weight, which generally has a size of 1 mm or less. Therefore, it has been desired to provide a method for stably producing a very high quality aromatic polycarbonate on a commercial scale at a rate of 1 t/hr or more for a long period of several thousand hours or more (for example, a period of time as long as 5,000 hours or more).